The present invention relates to a ball joint which prevents decrease in the frictional force between a stud ball and a ball seat.
Conventional ball joint is shown in FIG. 1, which comprises a ball stud 1 with a stud ball 2. The stud ball 2 is press-fitted into a spherical recess 4 of a ball seat 3 formed of synthetic resin, e.g. nylon. The ball seat 3 is then tightly fitted into a ball socket 5 and held in the ball socket 5 by an end cover 6 which is secured to an end opening of the ball socket 5 by calking. That end opening of the ball socket 5 through which the ball stud 1 extends is covered by a dust cover 7 which forms an inner space 8 filled with lubricating grease, and which hermetically seals the interior of the ball socket 5.
A ball joint achieves the desired function by a sliding movement of the stud ball 2 relative to the ball seat 3. The sliding surfaces of those two members are liable to wear during the operation. Further, the ball seat 3 tends to deform permanently due to the load applied by the ball stud 1. Thus, the frictional force generated between the stud ball 2 and the ball seat 3 decreases with time gradually as shown in FIG. 6 at a. When the frictional force decreases beyond a certain limit from the desired value b, the ball joint for a vehicle, for example, generates a disadvantageous effect known as shimmy.
Consequently, the conventional ball joint mentioned above cannot be used for a long period, and has to be replaced by a new joint in a relatively short period requiring frequent maintenance operations.